Adsorption of Humic Acid by Acid-Modified Granular Activated Carbon and Powder Activated Carbon

Activated carbon adsorption experiments were carried out with humic acid solutions to determine adsorption isotherms and kinetics. Four commercial coconut shell–based activated carbons were used: untreated, nitric acid–modified, sulfuric acid–modified, and base-treated. Adsorption capacities for humic acid were appraised for each type. Nitric acid and sulfuric acid modification increased humic acid adsorption capacity, as compared with untreated and base–modified activated carbon. Humic acid adsorption capacity for nitric acid– and sulfuric acid–modified powder activated carbon (PAC) was determined to be 50 and 70 mg/g, respectively. Adsorption capacity for the untreated carbon and the base-treated PAC was 28.7 and 7.6 mg/g, respectively. The zeta potential (δ) of activated carbons indicates that when acid modified they exhibit a net positive charge due to numerous carboxylic acid functional groups and positively charged hydroxyl groups attached to the surface, ultimately enhancing humic acid adsorption. Scanning electron microscopy (SEM) images confirmed that acid treatment of activated carbon enlarges its micropores and removes contaminants embedded in them. The Langmuir isotherm model and pseudo-second-order model provided the highest correlation coefficient for adsorption and kinetics.